Plasma start-up using transient CHI on NSTX

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Summary form only given. Future toroidal magnetic confinement fusion plasma devices such as the Component Test Facility (CTF) require toroidal current drive. Present devices rely on a solenoid to drive current, however, for devices with a high neutron flux, it is impractical to provide the necessary shielding. An alternative method must be provided both to provide toroidal current. A new method of non-inductive startup, referred to as transient coaxial helicity injection (transient CHI), was successfully developed on the Helicity Injected Torus (HIT-II) experiment. In this method, a plasma current is produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields chosen such that the plasma rapidly expands into the chamber. When the injected current is rapidly decreased, magnetic reconnection occurs near the injection electrodes, with the toroidal plasma current forming closed flux surfaces. In HIT-II CHI produced 100 kA of current on closed flux surfaces using 30 kA of injector current. Transient CHI has been successfully used on the National Spherical Torus Experiment (NSTX) for an unambiguous proof-of-principle demonstration of closed-flux current generation without the use of the central solenoid. When coupled with the HIT-II results, this demonstrates very favorable scaling to a larger device. The current multiplication factor between the injected current and the achieved toroidal current is 60 in NSTX, compared to six in HIT-II. Detailed experimental measurements made on NSTX include, fast time-scale visible imaging of the entire process. Experimental results from these experiments will be presented